This invention relates to an improved composition for providing corrosion resistance for corrodible metallic surfaces.
Various compositions have been proposed in the prior art to provide varying measures of protection against corrosion of corrodible metallic surfaces. In more recent years, attention has been progressively given over to providing compositions and systems for providing cathodic-anodic corrosion protection for metallic surfaces of structural elements. Such structures as underground pipes, storage tanks, buildings and the like, as well as metallic structures continually in contact with water, such as ships, support structures for drilling rigs, docks and the like have been treated with or coated with a variety of compositions or systems to impart varying degrees of corrosion resistance thereto.
Generally, such systems or compositions as have been employed utilize either an external source of electrical current which serves to maintain as cathodic the surface to be protected or the corrosion protection composition itself forms an internal current with the metallic surface to be protected. In the latter systems, the coatings contain metallic particles which are more anodic in nature than the metallic surface to be protected and thus serve to function as sacrificial anodes. Most commonly, in coatings of this latter type, the composition is comprised primarily of a binder and a filler. The binder may be any suitable organic or inorganic binder material and the filler is constituted by conductive metallic particles which are more anodic than the metallic surface to be protected against corrosion. Most generally, the metallic particles utilized in such coating compositions are zinc particles.
It is known that such zinc-rich (80-95%) coating compositions serve to protect against harmful corrosion of corrodible metal surfaces to a greater degree than do ordinary paints, particularly when applied to iron and steel surfaces. Such zinc-rich coating compositions are effective in salt-air atmospheres and in applications where the coated metal surface contacts brine solutions. Zinc powder has for some time been recognized as having a specific use as a pigment in the anti-fouling and anti-corrosive ship bottom paints. For such applications, it is necessary to produce coating compositions comprised of a finely divided zinc powder suspended in a heavy-bodied drying oil, a spar varnish or lacquer. Extreme fineness has been a chief requisite for successful application in this area.
Zinc-rich coating compositions are compounded in such a manner to provide a subsequent dry film containing 85 to 95 percent zinc dust, with no zinc oxide. The coatings are rich in metallic zinc which is in intimate contact with the iron or steel surface to be protected and provide sacrificial electrochemical or cathodic protection to the base metal, such as occurs on galvanized products. Also, as in galvanizing, the zinc-rich coating film is electrically conductive. This dual property of direct metal to metallic zinc contact and good electrical conductivity of the dry film distinguishes zinc-rich coatings from other corrosion-inhibitive coating compositions. The coating composition is formulated with the zinc dust in suitable vehicles, such as, for example, plasticized polystyrene, chlorinated rubber, and some inorganic materials. Such vehicles must have sufficient strength to carry the high metallic particle concentration involved and yet provide adherence and flexibility.
While such protective coating compositions have been used to an appreciable extent, the metal particles, such as zinc powder or dust, add greatly to the cost of such compositions. In addition to the cost factor, it is frequently necessary to utilize appreciable quantities of this relatively expensive metallic filler on the order of 80 percent or more of the weight of the total composition in order to provide the requisite corrosion protection of the metallic surface.
Attempts have been made to reduce the requirements for zinc metal in such zinc-rich coating compositions as, for example, by replacing a portion of the metallic zinc particle filler material with inert refractory alloys or the like. Such coating compositions, which are disclosed in U.S. Pat. No. 3,562,124, are electrically conductive, brittle, and substantially non-reactive in water. These compositions contain generally from 25 to 50 percent of the inert refractory ferroalloy and 50 to 75 percent zinc powder or dust, based on the total weight of filler present in the coating composition. While such compositions serve to impart corrosion resistance, such compositions still require the presence and use of considerable percentages of zinc powder or dust.
Zinc is in large measure imported into the United States and it is desirable to replace it with more commonly available metals.